Top Democratic lawmakers who opposed optional 401(k) pension plans for new, unionized state workers may soon be able to get the popular retirement option for themselves, The Post has learned.

Gov. Cuomo’s newly approved Tier 6 pension reform — designed to save state and local governments $80 billion over 30 years — allows new nonunion employees earning $75,000 or more to join a “defined contribution’’ 401(k) plan like those common in the private sector.

Sources told The Post the option certainly applies to new state Assembly and Senate members elected in November — and may even apply to re-elected current members of the Legislature, who technically start new terms of office after each two-year election cycle.

The legal concept that legislators start as new state workers was recognized in a law — repealed for new Assembly and Senate members in 1995 — that has allowed re-elected pols like Albany Assemblyman Jack McEneny to “retire’’ from the Legislature, collect a $73,000 annual pension, and then start a new term in office after being re-elected and receive a $94,500 annual salary.

It’s unclear how the law stands on 401(k) plans, though.

New York’s 212 lawmakers receive a “base’’ salary of $79,500 annually — many make much more — and they are, of course, nonunion.

“Everyone involved in the [Tier 6] negotiations recognized that new legislators will be eligible for the new 401(k) option,’’ said a negotiator who worked on the new pension bill. “The only question is: Will legislators currently in the state pension system be able to participate as well?’’

The question will first be answered by state Comptroller and pension-fund sole trustee Tom DiNapoli, a strong ally of the public-employee unions, who bitterly opposed the 401(k) option.

Should DiNapoli block new lawmakers from participating in a 401(k) option, a lawsuit is possible.

Meanwhile, a prominent Capital District assemblyman said he was shocked when a public-employee union lobbyist told him that unions didn’t want a 401(k) option “because a lot of members would take it.”

“He said, ‘We believe we know what’s best for our members, more than they do,’ and I was quite surprised by that kind of arrogance,’’ said the assemblyman, who demanded anonymity.

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An unspoken factor in Cuomo’s decision last week to accept the Senate district lines that were gerrymandered by the majority Republicans was the sorry state of the Senate’s Democratic leadership, administration insiders said.

Senate Minority Leader John Sampson (D-Brooklyn) and his senior leadership are widely seen as incompetent, out of touch with much of the state, and borderline corrupt, a situation that chilled Cuomo from fighting on their behalf.

“If the Senate Democrats had impressive credentials and serious leadership, if they genuinely stood for real reform and had high principles, I think the governor would have taken a different approach,’’ said a source close to the Cuomo administration.

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The Siena College poll earlier this month that showed a slight drop in Cuomo’s sky-high approval rating had much worse news for Attorney General Eric Schneiderman.

Just 30 percent of voters viewed his job performance favorably, and 43 percent rated it negatively.